In the process automation industry, field devices are often applied which serve for registering and/or influencing process variables. Examples of such field devices are fill level measuring devices, pressure and temperature measuring devices, pH measuring devices, conductivity measuring devices, valve controls, etc., which, as sensors, register, or, as actuators, control process variables such as fill level, flow, pressure, temperature, pH value or conductivity.
A large number of such field devices is produced and sold by the firm Endress+Hauser©.
Field devices are frequently connected with superordinated units, e.g. process control systems or controllers. These superordinated units serve for process monitoring, process control or process visualizing.
Signal transmission between field devices and superordinated units frequently occurs according to the 4 to 20 mA standard, via a two-conductor current loop. If the field devices in question are sensors, the measured values registered by them are transmitted as a direct current signal via the two-conductor current loop to the superordinated units. The measuring range of the sensors is, in such case, linearly mapped to a 4 to 20 mA direct current. Normally, this direct current signal is produced not in the superordinated unit, but instead in a separate measurement transmitter supply device, which is connected with the two-conductor current loop.
Intelligent field devices partially possess extensive diagnosis or configuration options, which, for example, concern information regarding the maintenance state of sensors, enable parametering the measuring ranges or indicate to the user an imminent failure of the device.
Such diagnostic information cannot easily be encoded in a 4 to 20 mA direct current signal. A solution which enables such diagnosis or configuration options while maintaining the physical, two-conductor, electrical current loop wiring can be developed with the use of digital communication. A widespread standard for digital communication via a two-conductor current loop is the so-called “HART” standard (acronym for “Highway Addressable Remote Transducer”).
In the case of this standard, signal transmission is carried out via the current loop in a frequency F-multiplex operation (see also FIG. 2). In the frequency band under 30 Hz—the so-called “analog band”, the measured variable is encoded in an analog manner via a direct current signal between 4 and 20 mA. Compatibility with existing pure analog sensors without diagnosis or maintenance functionality is therewith assured. The frequency band between 100 Hz and around 10 kHz is utilized for the transmission of digital data. These frequencies are achieved via an alternating current signal superimposed on the direct current signal. Two standards for use of this frequency band are in use: Bell 202 with a baud rate of 1200 Hz and a PSK modulation with a baud rate of 4800 Hz. In spite of the higher physical transmission rate, PSK modulation is often not used today due to the higher technical difficulty in the case of the modulation and demodulation.
For the use of field devices in an explosion-endangered area, in which precautionary measures against ignition sparks are imperative, measures regarding operational safety are required. A measure for assuring operational safety is to galvanically isolate electrical circuits in the measuring transducer or measurement transmitter.
A negative of galvanic isolation is the increased circuit complexity. It is especially necessary to transmit the measured variable from the measuring transducer across the galvanic barrier into the current loop, which is galvanically coupled with the superordinated unit. Apparatuses for this transmission are already known from the state of the art, e.g. from DE 698 35 808 T2.
The advantages of the galvanic barrier stand in opposition to the fact that the latter also forms an impediment for diagnostic information. Thus, for example, an error occurring in the current loop is not directly communicated with the field device. Should, for example, the electrical current flowing in the current loop be monitored by the electronics of the field device, it is necessary to provide apparatuses for transmission of a control variable across the galvanic barrier, which enables conclusions concerning the electrical current actually flowing in the current loop. Such control functions are of especially high importance for safety-critical applications, in the case of which the field device must detect malfunctions. Such control functions are, among other things, prescribed by standards such as SIL2 (acronym for “Safety Integrity Level 2”) for safety-critical applications.
If analog measuring signals are transmitted between galvanically isolated regions, these can be corrupted by aging of the coupling elements. For this reason, it is advantageous to transmit exclusively digital signals between the galvanically isolated regions. The costs caused by the galvanic isolation increase with the number of lines required therefor. A minimizing of the number of lines connecting the regions galvanically isolated from one another is, consequently, to be strived for.